In a wireless communications system, such systems comprise a network infrastructure and user equipment, which can for example be portable communications devices. Such communications devices typically receive and transmit signals through the same antenna or antennas. This means that some form of duplexing scheme is required in order to allow the device to separate the incoming and outgoing signals such that the former is not swamped by the latter. In this respect, Time-Division Duplexing (TDD) and Frequency-Division Duplexing (FDD) are both well-known duplexing schemes.
So-called 4G, or Long Term Evolution (LTE), is the successor to existing 2G and 3G communications systems. Both TDD and FDD variants of LTE-compliant networks are already in operation in many countries. In relation to radio spectrum availability, for historical reasons, there are 38 LTE operating frequency bands for the LTE standard as defined in the 3G Partnership Project (3GPP) Rel 11 of the LTE standard, of which 26 require FDD operation.
In FDD radio operation, there are two separate carriers at different frequencies, one for the uplink transmission and one for the downlink transmission. Isolation between the downlink and the uplink transmissions is usually achieved by transmission/reception filters called diplexing filters (duplexers or diplexers). These filters are typically implemented as two highly selective filters, one centred on a receive frequency band, the other centred on the transmit frequency band to separate the transmit signals from the receive signals and so prevent the transmit signals from interfering with the receive signals. Acoustic resonator filters, such as Surface Acoustic Wave (SAW) filters, are typically used to provide the low insertion loss and sharp roll-off required of duplexing filters. Although these are individually small and cheap, a communications device that is to support multiple frequency bands requires one duplexing filter per frequency band to be supported and further Radio Frequency (RF) switching for selection between the frequency bands so that the duplexing filters can share the antenna.
Furthermore, these filters cannot be integrated with a CMOS circuit owing to the high-Q resonators used to build SAW filters that require a separate material and manufacturing process and so they must be implemented off-chip. This is not usually problematic for a simple radio transceiver operating on a single frequency band. However, modern radio transceivers are usually multi-band. As mentioned above, the LTE standard currently specifies 26 FDD frequency bands. To support all of the specified frequency bands would require a manufacturer of user equipment to use multiple filters due to the need for one duplexing filter per frequency band supported. A bank of discrete duplexers is one known approach to providing the switching for selection mentioned above, the bank being connected to an antenna, transmitter and receiver via a multi-way RF switch, which selects the appropriate duplexer based upon a required frequency band of operation. Such an approach increases the complexity of the user equipment, as well as increasing the overall size and cost of the multi-band transceiver. This approach can also lead to performance penalties; for example, the introduction of the RF switch results in signal power losses as multiple frequency bands are supported.
Many device manufacturers simply circumvent this problem by designing and manufacturing differently configured devices supporting different sets of frequency bands of operation. Manufacturers thus provide a range of devices each of which is operable in different groups of territories with different frequency band combinations. It can therefore be appreciated that obviating the need for the above-described filters would remove a barrier to the manufacture of a “world phone”, the benefits of which would provide economies of scale to the mobile telephony industry, and mitigate an inconvenience for the international traveller.
Therefore, there is a significant market demand for a solution that is able to replace the fixed tuned duplexing filters with a flexible device that can support multiple, preferably all, frequency bands.
Although it is possible to tune duplexer filters making up a duplexer dynamically, such an approach is currently technically impractical because very high Q-factor resonators are required to achieve the desired selectivity and low power loss. Currently, in order to achieve the small filter size required, such resonators are only realisable as acoustic resonators, which have a well-known bi-resonant characteristic that limits their electrical tuning to only a small frequency range.
An alternative duplexing solution is the use of Electrical Balance Isolation (EBI) duplexers or so-called hybrid junction or hybrid circuits. This is a 4-port network that can separate the forward and reverse wave directions in a transmission line. Hybrid junctions can be made in a number of ways, including using transformers, waveguides (“magic tees”), or microstrips (“directional couplers”). Hybrid junctions can also be made using active circuits, as is the case for modern electronic analogue wireline phones.
The hybrid junction typically comprises a first (transmit) port, a second (antenna) port, a third (receive) port and a fourth (balance) port. In operation of an ideal hybrid junction where all ports are terminated with matched impedance loads, all power incident at the transmit port is divided between the antenna port and the balance port while zero power appears at the receive port. Likewise, all power incident at the antenna port is divided between the receive port and the transmit port, while zero power appears at the balance port. This manner of operation corresponds to the ideal hybrid junction exhibiting a 3 dB or half power coupling loss when in a transmit mode and in a receive mode.
Broadband hybrids can be made using transformers, and single-transformer circuits, for example as described in “A Multiband RF Antenna Duplexer on CMOS: Design and Performance” (M. Mikhemar, H. Darabi, and A. A. Abidi, IEEE Journal of Solid-State Circuits, vol. 48, pp. 2067-2077, 2013).
A theoretical hybrid junction with balancing network, when used as a duplexer, has a power amplifier of a transceiver transmitter chain coupled to the transmit port thereof and a low-noise amplifier of a transceiver receiver chain coupled to the receive port thereof. Transmit power applied at the transmit port by the power amplifier is, as described above, divided between the antenna port and the balance port and the low-noise amplifier is isolated, i.e. there is no leakage of a transmit signal into the receiver chain as long as the reflection coefficients at the antenna port and the balance port are identical in magnitude and phase, the reflection coefficients being dependent upon the impedance coupled to the antenna port and the impedance coupled to the balance port, respectively.
In practice, however, use of the hybrid junction with balancing network as a duplexer suffers from a number of drawbacks. Firstly, the impedance of the antenna, and so by extension the impedance at the antenna port, typically exhibits variation in both the time domain and frequency domain. The impedance of the antenna can vary with time, for example owing to objects moving in the proximity of the antenna, and consequently, it is necessary to adapt dynamically the impedance at the balance port to the impedance at the antenna port to account for these changes. The antenna impedance also typically varies with frequency and so, to obtain balance at the particular frequency of interest, the impedance at the balance port must be adapted accordingly, and a good balance may not be achievable over a sufficiently wide system bandwidth, for example the 20 MHz needed for an LTE channel.
Secondly, other coupling mechanisms cause leakage of some of the transmit signal from the transmit port to the receive port of the hybrid junction. As such, isolation of the receive port from the transmit port is limited.
Despite the above-mentioned drawbacks associated with use of the hybrid junction as a duplexer, attempts have been made to obviate or at least mitigate the above disadvantages. For example, “Optimum Single Antenna Full Duplex Using Hybrid Junctions” (Laughlin, Beach, Morris and Haine, IEEE Journal of Selected Areas In Communications, Vol. 32, No. 9, September 2014, pages 1653 to 1661), considers an arbitrary antenna with an impedance that can vary widely with frequency and with a return loss that is likely to be of the order of 10 dB minimum (as long as there are no de-tuning proximity effects). This is a practical reality for a transceiver circuit that can be built into a wide range of end products and could possibly be connected through an unknown length of transmission line. So-called Electrical Balance (EB) of the hybrid junction is proposed in the above-referenced document. This is one of a number of solutions that have been proposed in order to isolate the transmit port of the hybrid junction from the receive port.
Although advances have been made in hybrid junction duplexing design to minimise leakage of in-band transmit signals from the transmit port to the receive port, technical challenges nevertheless exist. For example, components of noise generated by transmit chain circuity but residing in the receive band of frequencies, for example by a synthesiser and/or a power amplifier, are still known to leak from the transmit port to the receive port of the hybrid junction and so are present in the receive band of receive chain circuity of a transceiver.
US patent publication no. 2007/0264943 is directed to a different approach with the objective of avoiding the need of an external SAW filter. Indeed, this document does not assume the need for a hybrid junction, rather it relates to an alternative to using the SAW filter. In this respect, the document relates to a method and apparatus to filter a so-called blocker signal frequency component from a signal comprising desired signal frequency components and the blocker signal frequency components. The solutions proposed comprise a filter arrangement applied across a Low-Noise Amplifier (LNA) of a receiver circuit. The circuit shunts a portion of the signal as applied to an input of the LNA and the filter arrangement removes the blocker signal frequency components from the shunt signal before combining the filtered shunt signal with an amplified signal output by the LNA. The combination results in the subtraction of the filtered shunt signal containing the blocker signal frequency components from the amplified signal output by the LNA, which comprises amplified blocker signal frequency components and desired signal frequency components, to leave the amplified desired signal frequency components for further processing by the receiver circuit. However, the “SAWless receiver” relies on the blocker signal frequency components and the desired signal frequency components being in different frequency bands, which differs from the challenges faced in relation to the leakage to the receive port of the hybrid junction of components of noise generated in the receive band of frequencies by the transmit chain circuitry that then appear along with the frequency components of interest in the receive band. In this respect, this is analogous to the blocker signal of US patent publication no. 2007/0264943 being in the same band of frequencies as the desired signal. Furthermore, the filter arrangement is applied across the LNA and attempts to filter out frequency components that would already be removed by the hybrid junction with balancing network were it to be employed.